The present invention relates to a method and an apparatus for treating fiber suspension. The method in accordance with the invention is especially suitable for screening pulps of the pulp and paper industry and also for thickening. The apparatus part of the invention relates to a rotor construction of a power screen or a thickener.
There are, in principle, two commonly used types of rotor arrangements to maintain the filter surface clean, in other words to prevent the generation of a fiber matting on the filter surface. An example of one of these types is shown in U.S. Pat. No. 4,193,865. This discloses a rotor arrangement, in which a rotatable rotor is arranged inside a cylindrical, stationary filter cylinder. The rotor comprises blades located close to the surface of the filter cylinder, the blades forming an angle with the axis of the cylinder. The filter surface is subjected to pressure pulses when the blades move; the pressure pulses serving to open the cylinder openings. There are also arrangements in which the blades are located on both sides of the filter cylinder. In such a case the suspension being treated is introduced either to the inside or to the outside of the cylinder, and the discharge of the accepts takes place on the outside or the inside respectively.
An example of the second type of rotor is shown in U.S. Pat. No. 3,437,204. There the rotor is a substantially cylindrical, closed piece, the surface of which has almost hemispherical protrusions. The pulp in this type of apparatus is introduced to the treating space between the rotor cylinder and the filter cylinder outside the rotor cylinder, whereby the purpose of the rotor protrusions (bumps) is both to press the pulp against the filter cylinder and to draw the matted pulp with the trailing edge from the openings of the filter cylinder. Because this type of construction has a highly thickening effect on the pulp, three dilution water conduits are provided at different levels in the filter cylinder so as to satisfactorily carry out the screening of the fiber suspension. A corresponding "bump rotor" is disclosed also in U.S. Pat. No. 3,363,759, in which the rotor is slightly conical. Various forms of protrusions may be utilized.
German application 3006482 discloses a knotter, which has on the surface of a cylindrical rotor drum plough-shaped protrusions made of plate material, the purpose of which is to create strong mixing forces in the pulp between the rotor and the filter cylinder so that the fibers penetrate the filter cylinder as effectively as possible, and the knots, chips and the like are separated.
U.S. Pat. Nos. 4,188,286 and 4,202,761 illustrate a filter apparatus, which has a rotatable cylindrical rotor inside the filter cylinder. protrusions are arranged on the rotor surface on the filter cylinder side, and the protrusions shaped in such a way that one rotational edge has an evenly rising front surface, a surface parallel to the rim of the rotor and a back surface substantially perpendicular against the rotor surface. These protrusions are arranged on the surface of the rotor cylinder in a particular angle position relative to the axial direction so that all the protrusions of the rotor are at a similar position relative to the axis of the rotor. According to these US patents, pulp can be introduced into this apparatus at either side of the filter cylinder. If pulp is introduced to the outside of the filter cylinder and the accepts are discharged from the inside of the filter cylinder, the rotational direction of the rotor is such that the accepts are subjected to a downwardly inclined force component by the angle position of the protrusions and that said inclined/rising surface of the protrusions operates as a front surface. If the pulp is introduced between the rotor and the filter cylinder, in other words the accepts are discharged from the outside of the filter cylinder, the rotational direction is opposite to that previously described, the protrusions tending to slow down the downwardly flowing pulp, and the surface which is perpendicular relative to the surface of the rotor cylinder operates as the front surface.
Practical industrial experiments have, however, proven that the above described apparatus does not operate satisfactorily in all application conditions. For example, the first mentioned blade rotor creates pressure pulses at the accept side of the filter cylinder that are too strong, and is not therefore suitable, for example, in head boxes of paper machines where any fluctuation of pressure is undesirable. The apparatus also tends to dilute the accepts, and therefore the blade rotor is not applicable in apparatus, which require pulp of a constant consistency. Since the blades, (4 to 8 blades) in the blade rotor are spaced relatively far from each other, a fiber matting always accumulates on the surface of the filter cylinder before the next blade scrapes it off. Thus the use of the filter is not effective. Additionally, this rotor type is expensive to manufacture due to accurate shapes of the blades and a careful finishing.
A substantially cylindrical rotor, which has almost hemispherical protrusions, operates almost ideally in some applications, but, for example, in the head box of a paper machine its operation can be provided only with additional preconditions. Because the pulp suspension flowing to the head box should be homogeneous, both in consistency and in fiber size, the power screen should not vary these values. However, this kind of a "bump rotor" tends to dilute the accepts, and it causes fluctuation in the consistency values. In experiments it was discovered that a rotor of this type diluted accepts between -0.15 to -0.45% when the accept consistency was 3%. Consequently, the consistency varies absolutely .+-.5%, which is too much when a homogeneous and qualified final product is desired. Fractionation also takes place in the screen comprising a "bump rotor", in other words the mutual relation between the fractions of the fiber suspension supplied to the filter cylinder changes in the screen in such a way that the relation of the fractions of the accepts is no longer the same as that of the originally supplied pulp. With a "bump rotor" the extent of change of said fractionation was experimentally found to be between 5 to 10% according to the clearance of the filter cylinder and the rotor. The corresponding extent of change with a blade rotor was about 20, and therefore even a "bump rotor" is a considerable improvement to the previous apparatuses.
These defects of a filter apparatus provided with a "bump rotor" described above have led to some improvement attempts, such as guidance of the dilution water to the filter surface, and in a slightly conical shape of the rotor. Both alternatives reflect the problem occurring in communication with the cylindrical rotor, i.e. the uneveness in the use of a filter cylinder in its different zones. The fact is that the flow through the filter cylinder is at its greatest immediately after pulp has come into communication with the cylinder and the rotor. Consequently the pulp thickens to some extent and when the pulp flows downwardly along the surface of the filter cylinder, the amount of the suspension flowing through the filter openings continuously diminishes. Attempts have been made to prevent this by feeding dilution water to different levels of the filter surface, which results in a slightly more effective operation of the filter cylinder, but which also results in the disadvantage of a relatively high dilution of the accepts. Another possibility is to vary the clearance between a filter cylinder and a rotor, whereby a greater clearance in the upper part of the filter apparatus enables a higher speed of the downwardly flowing pulp, so that the pulp fills the clearance better and more homogeneously.
An operating method is disclosed in the U.S. Pat. No. 4,188,286, in which the protrusions are inclined relative to the axis of the filter cylinder. The main purpose of the inclination is to prevent the fiber or fiber flocs from attaching to the front edge of the protrusion and from being conveyed with it. A secondary purpose is to bring the accept pulp in the treatment space between the rotor and the filter cylinder subject to a downward force component, by which it is possible to accelerate to some extent the operation of a filter apparatus, at least to accelerate the discharge of the accept from the filter.
The most developed embodiment in the market at the moment is illustrated by the method in accordance with FI patent 77279, and by the apparatus developed to realize this method. The method in accordance with this Finnish patent is characterized in that fiber suspension is subjected to axial forces, the intensity and the direction of which vary according to the mutual axial position between the point of application and the counter surface of the filter cylinder, and which are utilized to change the axial speed profile of the fiber suspension while maintaining the direction of the flow continuously towards the discharge end. The apparatus thereof is characterized in that at least one of the counter surfaces of the filter cylinder has at least one bump or the like, the direction of the front surface to which varies according to the axial position of the bump and by which the pulp particles in the space between the counter surfaces, i.e., the filter cylinder and rotor, are subjected to an axial force component, the intensity of which varies in function of the axial position of the pulp particles, and which changes the speed profile of the fiber suspension flowing between the counter surfaces.
Although the arrangement and the method in accordance with Finnish 72279 are superior to the prior art techniques, it is still possible to further develop the method and apparatus described therein. Minute experiments have shown that all rotors, which use any kind of protrusions (whether bladelike, hemispherical, rectangular or any other type),have associated therewith--beginning from the peak of said protrusion towards the trailing direction--pulp whose consistency and reject content is higher than in the screening zone. This is due to the fact that the pressure stroke caused on the pulp by the bump has pressed acceptable material through the filter surface, whereby both liquid and acceptable fiber material flows through the screen surface.
It has also been discovered in such experiments that said pulp which has a higher consistency and which contains more rejectable material tends to remain against the screen surface regardless of the fact that the effect of the bump on the rotor on said pulp. portion stops. This, of course, weakens the capacity of the screener, because fresh or less screened pulp must first penetrate the layer of a higher consistency in order to pass the filter. In thickeners, the fiber matting accumulated on the filter surface causes a situation to exist whereby in order for the filtrate to pass the opening of the filter surface it must also be pressed through the fiber matting.
The present invention relates to a method and apparatus for conveying the above mentioned thicker and coarser pulp portion from the filter surface towards the surface of the rotor so that the fresher pulp comes into direct communication with the filter surface, whereby the disadvantages of the prior art apparatus discussed above may be minimized or eliminated.
The method in accordance with the present invention is characterized in that the pulp fraction concentrated adjacent to the filter surface and/or containing coarser material is subjected to a force component directed away from the filter surface, by means of which that fraction is conveyed away from the filter surface.
An embodiment of the apparatus in accordance with the present invention is characterized in that at least one guide plate is arranged to communicate with the member counter to the filter surface, i.e., the rotor, which guide plate guides the coarser and/or thicker suspension concentrated adjacent to the counter member surface, away from the filter surface.
Another embodiment of the apparatus of the invention is characterized in that the counter surface to the filter surface has at least one member which is formed by a protrusion arranged on the counter surface, and a guide plate extends from the level of the counter surface higher than the protrusion. The protrusion and guide plate leave an opening therebetween, through which the thicker and/or coarser fraction can flow, under the guide plate.
The method and apparatus in accordance with the present invention are described more in detail below, by way of example, with reference to the accompanying drawings, in which: